Track cause of slow render
So I figured out how to make it myself and folks can find the script here:

Code on pastebin

If you have a very simple scene then you might get some wonky results, as this calculates based on render duration and every single time you render a frame (even the same frame), you'll get different times. The best use for this is on a semi-complex scene so that the various samples (diffuse, transmission, etc.) really get to take effect and modify the total render time.